Distillation columns are one of the most commonly used thermal separation units and have been used for many centuries. Their operation is based on the difference in boiling temperatures of the liquid mixture components, and on recycling counter-current gas-liquid flow. The properly organized temperature distribution up the column results in different mixture compositions at different heights. While multi-component is a common phenomenon for all column types, the flow regimes are very different depending on the internal elements used. The two main types are a tray column and a packed column, the latter equipped with either random or structured packing. Different types of distillation columns are used for different processes, depending on the desired liquid holdup, capacity (flow rates), pressure drop etc, but each column is a complex unit, combining many structural elements.